Thymoma is generally considered as an epithelial malignancy surrounded by lymphocytes not belonging to the tumour. This report documents a case of malignant thymoma associated with a lymphocytic pleural effusion. The pleural lymphocytes were mature T-cells, a small proportion of which were shown to have a monoclonal rearrangement of the beta-chain of the T-cell receptor. On the contrary, the lymphocytes of peripheral blood exhibited a germline configuration. The nature of the monoclonal population found in the pleural fluid is discussed and if the cells are of thymic origin their clonal configuration may point to a neoplastic nature of lymphocytes infiltrating malignant thymoma. Alternatively, he clonal T-cell subpopulation may result from an aberrant immunological response to the thymoma.